The present invention generally relates to reference standards and ultrasonic inspection methods and, more particularly, to a laser-etched porosity reference standard for ultrasonic inspection of composite materials and to an ultrasonic porosity standardization process.
The use of fiber-reinforced composite laminate structure as a structural element is rapidly increasing, for example, in the aerospace industry, due to the weight savings, which lead to a reduction in aircraft fuel consumption, and the improvement in fatigue life and corrosion control that can be achieved by using fiber-reinforced composite materials. Typically, fiber-reinforced composite materials contain a strong and stiff fiber, such as a carbon fiber, embedded in a softer matrix material, such as a resin. The resin is used as a binding agent to hold the fibers together while the fibers provide the strength. Laminated composites are constructed of many layers of fiber-reinforced materials. Fiber-reinforced polymer matrix composites, such as graphite/epoxy and carbon/cyanate ester are now the materials of choice for spacecraft and launch vehicle structures and subsystems such as optical benches, instruments, and antennas. Key components, such as wings, empennage and fuselage skins, for example, in commercial and military aircraft, are now being constructed almost exclusively of this material. Fiber-reinforced polymer matrix composites are also widely used, for example, in sports, industrial, and medical equipment. With more and more critical structure being made of fiber-reinforced composite laminates, ways to assure good quality construction are needed more than ever before. Consequently, inspecting these composite structures is a critical element in assuring the material quality needed for their application, for example in a commercial airplane.
Laminated composite materials undergo non-destructive testing procedures, such as ultrasonic and radiographic inspection, during aircraft manufacturing, maintenance, and repair. Common aircraft applications include, for example, porosity testing, thickness testing, and crack detection of the aircraft airframe. Ultrasonic testing introduces high frequency sound waves into the test material to detect subsurface discontinuities. Transducers are used both to transmit and receive sound energy into and from test material. In the process, high frequency sound in the order of 500 KHz (kilohertz) to 10 MHz (megahertz) is sent, for example, into a composite laminate and echoes from the laminate are then measured in the time domain and the amplitude domain to determine the materials quality. Through-transmission and pulse-echo techniques are most commonly used in the aircraft airframe industry, both commercial and defense. Some advantages of the ultrasonic testing method include high penetrating capability, high sensitivity and resolution, portability, single surface accessibility, and the immediate interpretation of test results.
Porosity is a known detrimental material condition that includes a plurality of voids in the composite material caused by trapped or evolved gases which may be caused by improper processing, such as improper curing of the composite material. Currently, millions of dollars are spent on ultrasonic systems designed to detect and quantify porosity in composite laminate parts manufactured, for example, from graphite/epoxy composites. The ultrasonic inspection of composite laminates takes advantage of the fact that porosity does not block ultrasound, like, for example, a delamination does, but scatters and attenuates the ultrasound. Consequently, by measuring the amount of attenuation an estimate of the degree of porosity can be obtained for correlation with manufacturing specifications. Typically, attenuation curves are produced to match ultrasonic attenuation on the inspected composite laminate part with actual porosity levels. Unfortunately, such attenuation curves are individual to the interrogating transducers and ultrasonic machine, and, therefore, different instrumentation at different inspection sites may produce different results. Consequently, the part might pass the specification tolerance at one inspection site but may fail at another.
To mitigate the above problem, sets of porosity reference standards are manufactured for use at each inspection site. These porosity reference standards are necessary to compare the ultrasonic response between the inspected part and a known porosity level tested with a particular ultrasonic machine. Typically, these sets of porosity reference standards include 30 to 40 panels, having a size of about 4 by 4 inches with varying thicknesses, for a variety of porosity levels. Currently, the panels are made out of the same fiber-reinforced composite material as the part to be inspected, for example, graphite/epoxy. Generally, during the manufacturing process of the panels, the cure parameters are altered to produce varying degrees of porosity. Since the porosity levels of a panel typically vary spatially due to the inherent anisotropy of the fiber-reinforced material, the ultrasonic attenuation values must be averaged over the area of the panel. Frequently, destructive testing, for example, by cross-sectioning, polishing and optical analyzing, of an area of similar attenuation values in the panel is performed so that the exact cross-sectional porosity content can be identified for correlation with the attenuation value. This is an arduous and expensive process. Since the number of suppliers of fiber-reinforced material parts that perform on site porosity inspection and evaluation of the manufactured parts is growing, it is necessary to manufacture more sets of porosity reference standards to distribute to each supplier, further increasing manufacturing cost and time.
As can be seen, there is a need for porosity reference standards that allow standardization of ultrasonic inspection equipment used for inspection of fiber-reinforced composite materials and enable a consistent porosity inspection process. Furthermore, there is a need for a lower-cost, reliable way of producing porosity reference standards. There has, therefore, arisen a need to provide a porosity reference standard that can be used in ultrasonic porosity inspection processes for composite laminates, that provides a reliable, common reference for porosity values, that enables consistent and accurate porosity evaluation of porosity levels in composite materials, and that can replace existing expensive composite porosity reference standards. There has further arisen a need to provide a method for producing voids within a material to manufacture a porosity reference standard, which has a constant level of porosity over an area, at a lower cost and with a reduced machining time compared to existing standards.